ALTERNATING GENERATIONS 



but Dr. Lang's drawing shows how, nevertheless, the prothalli in turn 

 hasten to a fresh apogamy (Fig. 38), thus the two transitions may be 

 repeated at near intervals of time. The same was seen to be the case 

 in Trichomanes alatum, in which apospory and apogamy were found to 

 succeed one another, 1 and it has recently been proved to hold also for 

 Athyrium filix foemina, var. clarissima^ Jones, and other Ferns. 2 



Lastly, Goebel has shown 3 that when the seedling leaves of certain 

 ferns are removed and cultivated on a moist substratum, aposporous 

 growths may be induced, which show sometimes the most intimate inter- 

 mixture of characters of the sporophyte and 

 the gametophyte. These developments appear 

 to be similar in kind, though not in detail, to 

 those described by Lang and others. It would 

 doubtless be possible to erect upon such facts 

 a superstructure of theory ; but it is necessary 

 to remember that by the abscision of a young 

 part it is placed in an anomalous and extreme 

 physiological position. It is improbable that 

 such circumstances ever arose in the course 

 of descent : and accordingly it must remain 

 a quite open question what bearing, if any, 

 such observations have upon the evolutionary 

 story. They demonstrate possibilities : but 

 possibilities are not the equivalent of historical 

 data. 



The rapid succession of the transitions 

 thus actually seen in some Ferns from the 

 sporophyte to the gametophyte, and the con- 

 verse, give some colour to the suggestion 

 made by Goebel, that the sporophytic buds 

 he found in the deep-water specimens of 



Isoetes are to be viewed as extreme cases of the telescoping of the 

 alternate generations. 4 This state of affairs is very nearly matched by 

 certain Adiantums observed by Lang, in which numerous sporophytic buds 

 were produced from the sorus. Examination showed that they sprang in 

 certain cases from the sporangia themselves, but not from the sporogenous 

 tissue. If we imagine the gametophyte stage reduced in such cases, not 

 to a very short phase only, as it is in Lang's Nephrodium, but to the 

 vanishing point, the result might be as in Goebel's Isoetes. But we may 



1 Ann. of Bot., vol. i., p. 269. 



2 Farmer and Digby, Ann. of Bot., 1907, p. 163-167. 



3 Sitz. d. Math.-phys., Klasse d. K. Bayer. Akad. d. Wiss., xxxvii., 1907. Heft, ii., 

 p. 119. This is interesting for comparison with my own negative results on leaves of 

 mature plants recorded in Ann. of Bot., iv., p. 168 (1889). 



*Bot. Zeit., 1879, P- I- 



FIG. 38. 





